1. Field of the Invention
The present invention relates to an optical parametric circuit which outputs a wavelength-converted light or a phase-conjugated light of an input signal light by using optical parametric effect of optical nonlinear mediums, and which can amplify these lights.
2. Description of the Related Art
In recent years, elements which can convert a wavelength of light without converting the light into an electric signal have been developed. There are following three kinds of elements as examples. These three kinds of elements are an element which performs four-wave mixing by a semiconductor optical amplifier or an optical fiber, an element which performs cross-gain modulation by a semiconductor optical amplifier, and an element which performs cross-phase modulation by semiconductor optical amplifiers.
As for four-wave mixing by the semiconductor optical amplifier, as shown in FIG. 1A, a signal light and a pump light are applied to a semiconductor optical amplifier 91 which is the optical nonlinear medium. Then, a wavelength-converted light (four-wave mixed light) is generated and is separated from the signal light and the pump light by an optical filter 92 and is output, in which the light frequency (2nfp-fs) of the wavelength-converted light and the light frequency (fs) of the signal light are symmetric with respect to the light frequency (fp) of the pump light.
As for the cross-gain modulation by the semiconductor optical amplifier, as shown in FIG. 1B, when a signal light of wavelength λs is applied to a semiconductor optical amplifier 93 which is rendered under gain saturation condition by applying a pump light of wavelength λp, gain for the pump light of wavelength λp decreases if the intensity of the signal light is high. Thus, the pump light of wavelength λp is output such that code represented by the pump light is logically inverted with respect to code represented by the signal light. Then, the pump light is separated from the signal light of wavelength λs by an optical filter 94 and is output as the wavelength-converted light.
As for cross-phase modulation by semiconductor optical amplifiers, as shown in FIG. 1C, a pump light of wavelength λp is divided into two lights by an optical coupler 95-1 which lights are applied to two semiconductor optical amplifiers 96-1 and 96-2. In addition, a signal light of wavelength λs is applied to the semiconductor optical amplifier 96-1 via an optical coupler 95-2 from the opposite direction, in which output lights from the two semiconductor optical amplifiers 96-1 and 96-2 are combined by an optical coupler 95-3. When the signal light is applied to the semiconductor optical amplifier 96-1, the refractive index of the semiconductor optical amplifier 96-1 is changed so that the phase of the pump light which passes through the semiconductor optical amplifier 96-1 is changed. Thus, the phases of the pump lights output from the two semiconductor optical amplifiers 96-1 and 96-2 become different. As a result, when the lights are mixed by the optical coupler 95-3, phase variation appears as intensity variation. Therefore, the pump light of wavelength λp which has the same logic code as that of the signal light of λs is output from the output edge of the optical coupler 95-3 as the wavelength-converted light.
Semiconductor devices used in the above-mentioned structures have a limitation of response speed. Thus, it is technically difficult and costs very much to process high speed signal higher than 40 Gbit/s.
In order to solve these problems, a wavelength conversion technique by using optical parametric process in a second-order optical nonlinear medium is proposed. The response speed of the second-order optical nonlinear medium is high such that wavelength conversion of ultrahigh-speed optical signal faster than 100 Gbit/s is possible. The wavelength conversion by using optical parametric process can be performed by a third-order optical nonlinear medium. However, it is known that, generally, the nonlinear coefficient of the second-order optical nonlinear medium is larger than that of the third-order optical nonlinear medium so that the second-order optical nonlinear medium can generate the wavelength-converted light efficiently by using short length crystal (reference: M. H. cho, et al., IEEE photonics technology letters, VOL. 11, pp. 653, 1999).
As for the wavelength conversion by using optical parametric process, conversion efficiency becomes higher when a traveling-wave type device is used and it is desirable that the wavelength conversion is performed in a configuration configured such that an input signal light and a wavelength-converted light are output in the same direction. However, according to this configuration, there is a problem in that, when wavelength difference between the input signal light and the wavelength-converted light is small, it becomes practically impossible to separate the input signal light and the wavelength-converted light at the output side.